During fetal development the testis descends into the inguinal canal and as it does so it pushes in front of it a covering of peritoneum (Fig. 6.1). This covering of peritoneum, which actually forms a tube, is called the processus vaginalis. The testis lies behind this tube of peritoneum and by birth, or shortly afterward, the lumen of the tube becomes obliterated. In the scrotum, the tube of peritoneum is called the tunica vaginalis. The testis essentially is pushed into the tunica vaginalis from behind. The tunica vaginalis, therefore, is actually two layers of peritoneum, which cover the testis everywhere apart from its most posterior surface (Fig. 6.2). The layer of peritoneum that is in direct contact with the testis is called the visceral layer of the tunica vaginalis, and the layer that surrounds this, and actually covers the inner surface of the scrotum, is called the parietal layer of the tunica vaginalis.

In the neonate, the parietal layer of the tunica vaginalis may not have firmly fused with the other layers of the scrotum, and therefore it is possible for the tunica vaginalis and the contained testis to twist within the scrotum. This is called an extravaginal torsion, i.e., the twist occurs outside of the two layers of the tunica vaginalis. In boys and men, the parietal layer of the tunica vaginalis has fused with the other layers of the scrotum. Thus, an extravaginal torsion cannot occur.